seymour wrote:i ended up buying a Gaggia Achille and Macap MC4 -- both refurbs so the cost was just over $1000 ...
i'm now trying to learn all the techniques for making an espresso that isn't bitter or sour.
There are plenty of discussions on H-B.com of how heat-exchanger (HX) machines like the Achille work, and in the review of the machine there are discussions of flushing regimens.
Some basic points to bear in mind:
The pressurestat sets the pressure inside the boiler, and pressure determines boiler temperature. Since water under pressure can exceed 212° F, the temperature of the water inside the boiler can be 20 or 30 degrees hotter than what you want for espresso. That's a huge number when you consider that a difference of only a couple of degrees can have a significant impact on espresso flavor. You want brew water to be approximately 200° F plus or minus about 6°. That's the entire target, not the bullseye. The bullseye changes from roast to roast, blend to blend, but the target remains pretty much the same.
On HX machines, the boiler water is not the brew water; the boiler water is for producing steam and for heating the brew water, which passes through the boiler in an enclosed tube. The temperature of the water inside that tube is superheated. The goal of the flushing is to empty the tube of the superheated water, so that the tube is filled with fresh cool water, and to give the boiler enough time to heat, but not to overheat, the new fresh water, taking into account the capacity of the group brass to draw some excess heat out of the water. A longer flush brings more cool water to the group, increasing its capacity to draw more heat from the water.
Which brings us back to the pressurestat. The higher the pressurestat setting, the less time it takes to superheat the fresh incoming water, because as pressure increases so does boiler temperature. To make your machine more forgiving -- to lower its temperature and lengthen that
rebound time-window-- you can lower the pressurestat.
In a domestic setting, when you're not entertaining but making a few drinks, a quick rebound is unnecessary and IMO undesirable. It
is important for catering, to keep up with milk-steaming demands of impatient customers, so bear that in mind when you see people praising a machine's quick rebound. No manufacturer wants their machine to get a reputation for anemic steam, so they often set the pressurestat higher than it needs to be for domestic duty.